Heat exchanger cores of the type with which the present invention is concerned, sometimes referred to as printed circuit heat exchanger (“PCHE”) cores, were developed initially by the present Inventor in the early 1980's and have been in commercial production since 1985. The PCHE cores are constructed most commonly by etching (or “chemically milling”) channels having required forms and profiles into one surface of individual plates and by stacking and diffusion bonding the plates to form cores having dimensions required for specific applications. Although the plates-and channel dimensions can be varied significantly to meet, for example, different duty, environmental, functional and performance requirements, the plates might typically be formed from a heat resisting alloy such as stainless steel and have the dimensions: 600 mm wide×1200 mm long×1.6 mm thick. The individual channels in the respective plates might typically have a semi-circular cross-section and a radial depth in the order of 1.0 mm.
Headers are mounted to the cores for feeding fluids to and from respective groups of the channels in the cores and, depending for example upon functional requirements and channel porting arrangements, the headers may be coupled to any two or more of the six sides and faces of the cores.
The design of PCHE cores or, more specifically, heat exchangers incorporating such cores requires the reconciliation of a number of (sometimes conflicting) considerations which, in the context of the present invention, include the following:    1. Achieving required thermal effectiveness (boundary temperatures) within allowable pressure drops,    2. Minimising the size and/or mass of the heat exchanger, and    3. Configuring a suitable shape for the core and/or porting arrangements for the groups of channels in a manner to facilitate the convenient connection of heat exchange fluids using conventional piping/coupling arrangements.
In researching approaches that might be made toward meeting these requirements the present Inventor has recently determined that, in order to achieve minimisation of the heat exchange area that is required in a given case to meet specified duty requirements, it is necessary to provide plate channels having high levels of tortuosity. However, channels that are configured along their lengths to provide high tortuosity must be made shorter than those having a lower level of tortuosity in order that pressure drop constraints might be met.
Shortening of the channels would not normally create a significant problem in the case of cross-flow heat exchangers. However, it would lead to a reduction in heat exchange/plate area utilisation in the case of the more usual co-flow and counter-flow heat exchangers which inevitably have at least some plates (typically between 50% and 100% of the total number of plates) that effectively incorporate cross-flow channels to direct inflow and outflow of fluid to and from orthogonally extending co-flow or counter-flow fluid channels. That is, if the length of the co-flow or counter-flow channels were to be reduced, the areas of the plates occupied by the cross-flow channels would increase relative to the area occupied by the co-flow or counter-flow channels. This would lead to the requirement for plates having a larger length-to-width ratio if the more usual area relativities were to be preserved and, given the requirement for shorter channels, to the need logically for smaller plates than those that customarily are used in the PCHE cores. This in turn would lead to difficulties with connection of heat exchange fluids using conventional piping/coupling arrangements.